In an internal combustion engine provided with a fuel injector, injected fuel is supplied to the cylinder as a spray from an intake port. However, as the fuel is originally in a liquid form, part of it adheres to the intake port or the intake valve, and it therefore enters the cylinder in a different form to that of the air-fuel mixture, so-called wall flow. The time required for fuel in this form to enter the cylinder is different from the time required for fuel in the air-fuel mixture to enter the cylinder, and the wall flow reaches the cylinder later than the fuel in the air-fuel mixture. If the engine is running under steady state conditions, this delay has no effect on the air-fuel ratio in the cylinder. However, under transient running conditions such as during acceleration or deceleration, the air-fuel ratio in the cylinder may fluctuate toward rich or lean with respect to the theoretical (stoichiometric) air-fuel ratio due to the difference in the rate at which the air-fuel mixture and the wall flow reach the cylinder.
More specifically, as some of the extra injected fuel during acceleration becomes wall flow, a delay occurs in the increase of fuel with respect to the increased air flowing into the cylinder, hence the air-fuel ratio shifts to lean. Conversely during deceleration, due to the wall flow, a delay occurs in the decrease of fuel with respect to the decrease of air flowing into the cylinder, hence the air-fuel ratio shifts to rich. Moreover, not all of the fuel particles in the air-fuel mixture flow uniformly into the cylinder and a part of them flows sluggishly along between the injector and the cylinder. These fuel particles are therefore delayed with respect to the air flow.
The actual amount of the delay in the fuel supply due to this fuel adhesion or sluggishness of fuel flow depends on, for example, the structure of the engine, the engine temperature and the structure of the gas flow passage. In this respect, Tokkai Sho 63-41634 published by the Japanese Patent Office discloses an engine fuel injection controller which corrects the fuel injection amount for a short-term, smaller delay having a relatively fast time constant, and a long-term, larger delay having a relatively slow time constant.
In this controller, a correction amount is determined according to the engine running conditions for the short-term delay and long-term delay respectively, the difference between the correction amount and a feedback correction amount determined from a measured air-fuel ratio is calculated, and the difference is learned so as to update the correction amount for both the short-term delay and the long-term delay.
The amount of fuel which is delayed also varies according to the temperature of that part of the intake port where the wall flow forms, and it increases the lower the temperature. The short-term delay which varies with a fast time constant contains elements such as a delay in computing the fuel injection amount which is unrelated to the wall flow, but almost all of the long-term delay which varies with a slow time constant is due to wall flow. This is why the long-term delay largely increases when the temperature of the fuel adhesion area decreases, whereas the short-term delay increases only slightly.
These flow delays also vary according to the nature of the fuel. For example, heavy gasoline has a low volatility so that wall flow forms easily. When the fuel is changed from standard gasoline to heavy gasoline, therefore, although both the short-term and long-term delays increase, it is the long-term delay which shows a particularly large increase.
Hence, by learning separate correction amounts for the short-term and long-term delays as described hereinabove, it is possible to allow for these air-fuel ratio fluctuations with different responses.
In the aforementioned fuel injection controller, the correction amount used in the learning process is stored together with the engine running conditions, i.e. the engine speed and load. However, as the amount of fuel which is delayed varies according to the temperature of the part where wall flow is formed, both of the aforementioned delays will vary if this temperature varies from the time of learning to the time when the learned value is used. In this controller, therefore, there was a problem in that temperature variation of the intake port where the injected fuel adheres lessened the precision of the correction, and in such a case a long time was required before the air-fuel ratio settled at the theoretical air-fuel ratio.